Hard abrasive particle-free polishing of hard materials

ABSTRACT

A method of CMP includes providing a slurry solution including ≥1 per-compound oxidizer in a concentration between 0.01 M and 2 M with a pH from 2 to 5 or 8 to 11, and ≥1 buffering agent which provides a buffering ratio ≥1.5 that compares an amount of a strong acid needed to reduce the pH from 9.0 to 3.0 as compared to an amount of strong acid to change the pH from 9.0 to 3.0 without the buffering agent. The slurry solution is exclusive any hard slurry particles or has only soft slurry particles that have throughout a Vickers hardness &lt;300 Kg/mm2 or Mohs Hardness &lt;4. The slurry solution is dispensed on a hard surface having a Vickers hardness &gt;1,000 kg/mm2 is pressed by a polishing pad with the slurry solution in between while rotating the polishing pad relative to the hard surface.

FIELD

Disclosed embodiments relate to chemical mechanical polishing (CMP) forpolishing hard material surfaces of semiconductors or on semiconductors.

BACKGROUND

Hard metal layers used in semiconductor wafer fabrication such astungsten, iridium, and ruthenium that have a Vickers Hardness exceeding1,000 Kg/mm² and typically do not react readily with chemicals such asoxidizers used in CMP resulting in a low CMP removal rate. Hardnon-metal layers having a Vickers hardness exceeding 1,000 Kg/mm² arealso typically non-reactive again leading to a low CMP removal rate.Examples of hard non-metals include diamond and some nitrides (e.g.,GaN, AlN or their mixtures), some carbides (e.g., SiC), some oxides ofelements in the Group III of the periodic table, as well as carbide,oxides and nitrides or mixtures thereof of metals in rows 3, 4, 5, 6 ofthe periodic table. Because of their hardness, materials having aVickers hardness greater than 1,000 Kg/mm² typically all require hardabrasive particles such as silica, alumina or diamond to enablepolishing with a reasonable polishing rate.

SUMMARY

This Summary briefly indicates the nature and substance of thisDisclosure. It is submitted with the understanding that it will not beused to interpret or limit the scope or meaning of the claims.

Disclosed embodiments recognize slurries having typical hard abrasiveparticles for polishing hard materials such as SiC, GaN and metals suchas ruthenium and tungsten can provide reasonably high polishing rates,but cause significant surface and sub-surface damage. Slurries withmoderately hard particles which are softer than such hard surfacestypically provide low polishing rates, with significantly less damage.However, since the moderately hard particles are still significantlyabrasive (having a Mohs hardness of about 6 to 10), crystal damage isgenerated during the CMP process. Moreover, since such hard materialsare generally chemically inert, the CMP process typically is very slowand thus requires a long cycle time as the slurry chemicals do not reactwith the hard surface. Therefore, there is a need to develop new CMPslurries and/or methods for polishing hard surfaces which decreasecrystal damage and increase the polishing rate as compared toconventional hard abrasive particles-based slurries.

Disclosed embodiments includes a method of CMP include providing aslurry solution including at least one per-compound oxidizer with a pHlevel from 2 to 5 or 8 to 11, and at least one buffering agent. Thebuffering agent provides a buffering ratio of at least 1.5 whichcompares an amount of a strong acid needed to reduce the pH of theslurry solution from 9.0 to 3.0 with the buffering agent compared to anamount of strong acid to change the pH of the slurry solution from 9.0to 3.0 without the buffering agent. The slurry solution is exclusive anyhard slurry particles or has only soft slurry particles that havethroughout a Vickers hardness less than 300 Kg/mm² or Mohs Hardness lessthan 4. A hard surface is defined herein as a material having a Vickershardness >1,000 kg/mm² is pressed with a polishing pad with the slurrysolution in between while rotating the polishing pad relative to thehard surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart that shows steps for an example method of hardabrasive particle-free polishing of hard materials, according to anexample embodiment.

FIG. 2 is a table showing experimental hard abrasive particle-freepolishing data results corresponding to Example 1.

DETAILED DESCRIPTION

Embodiments of the invention are described with reference to theattached figures, wherein like reference numerals are used throughoutthe figures to designate similar or equivalent elements. The figures arenot drawn to scale and they are provided merely to illustrate certainfeatures. Several aspects of this Disclosure are described below withreference to example applications for illustration.

It should be understood that numerous specific details, relationships,and methods are set forth to provide a full understanding of the subjectmatter in this Disclosure. One having ordinary skill in the relevantart, however, will readily recognize that embodiments of the inventioncan be practiced without one or more of the specific details or withother methods. In other instances, well-known structures or operationsare not shown in detail to avoid obscuring subject matter. Embodimentsof the invention are not limited by the illustrated ordering of acts orevents, as some acts may occur in different orders and/or concurrentlywith other acts or events. Furthermore, not all illustrated acts orevents are required to implement a methodology in accordance with thisDisclosure.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of this Disclosure are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contains certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements. Moreover, all ranges disclosed hereinare to be understood to encompass any and all sub-ranges subsumedtherein. For example, a range of “less than 10” can include any and allsub-ranges between (and including) the minimum value of zero and themaximum value of 10, that is, any and all sub-ranges having a minimumvalue of equal to or greater than zero and a maximum value of equal toor less than 10, e.g., 1 to 5.

Disclosed embodiments include a CMP slurry that is hard abrasiveparticle-free and a related method of hard abrasive particle-freepolishing of hard wafer surfaces having a Vickers hardness >1,000Kg/mm². The slurry comprises an aqueous solution medium including atleast one per-compound oxidizer, where the slurry lacks any hardabrasive particles that are conventionally required as known in the artof CMP for the polishing of hard materials. Hard abrasive particleswhich are excluded from disclosed slurries are defined herein as havingVickers hardness >500 Kg/mm² or Mohs hardness greater than 6, such assilica, alumina, diamond, or titania.

A per-based compound as used herein is a compound that includes anelement in its highest oxidation state. Some per-compound oxidizersinclude transition metal compounds, such as permanganate (MnO₄ ⁻), andsome non-transition elements such as perchlorate (ClO₄ ⁻). Optionally,the slurry solution can also include transition metal ions besides thosethat may be in the per-compound oxidizers in a concentration from 0.03 Mto 1 M, as well as optional chelating agents such as ethylenediaminetetraacetic acid (EDTA), or corrosion inhibitors such azoles and amines.

Examples of transition metal elements in disclosed per-compoundoxidizers include cerium, manganese, chromium, titanium, iron, cobalt,copper, zinc, nickel, and vanadium. Typical examples of per-compoundtypes include permanganate, peroxide, perchlorate, and persulfatecompounds. One particular per-compound type is an alkali metal (e.g.,sodium, lithium, potassium) of permanganate, or a mixture of aper-compound with one component being a permanganate.

Examples of specific per-compound oxidizers include PotassiumPermanganate (KMnO₄), sodium Permanganate (NaMnO₄), PotassiumPerchlorate (KClO₄), Potassium Periodate (KIO₄), Potassium Perbromate(KBrO₄), Potassium Peroxide (K₂O₂), Potassium Peroxoborate (KBO₃),Potassium Peroxochromate (K₃CrO₈), Potassium Peroxodicarbonate (K₂C₂O₆),Potassium Peroxodisulfate (K₂S₂O₈), Potassium Perrhenate (KReO₄),Potassium peroxymonosulfate (KHSO₅), Potassium Ortho Periodate (K₅IO₅),and Potassium peroxomonosulfate (or Peroxymonosulfate) (K₂SO₅). Theoxidation state of manganese in permanganate is +7, which is the highestoxidation state for manganese. Similarly the oxidation state forchlorine in chlorate is +7, which is its highest oxidation state. Theoxidation state of the transition metal or per-based oxidizer can be atleast +3, or higher. Examples of +3 or higher oxidation state transitionmetals include V^(3+,4+,5+), Ti^(3+, 4+), Cr^(3+, 6+), Mn^(+3+, 4+, 7+),Fe³⁺, Ni³⁺, Co³⁺, Mo^(3+, 4+, 5+, 6+), Ru^(3+, 4+), Pd⁴⁺, Ta^(4+, 5+),W⁶⁺, Re^(4+, 6+, 7+), Au³⁺, and Zr⁴⁺. A mixture of per-compoundoxidizers can also be used. The concentration of per-compound oxidizerscan vary from 0.01 M to 10 M or up to maximum solubility of theper-based compound at an elevated CMP temperature used (e.g., 70° C.),but is typically between 0.01 M and 4 M, such as between 0.1 M and 2 M.

Examples of hard metals for disclosed polishing include Ir, W, Ta, andHf. Examples of hard non-metal materials include carbides such as SiC,nitrides such as GaN, AlGaN, and AlN, diamond, and non-metals containingeither nitrogen, carbon or a mixture of both nitrogen and carbon. The pHof the slurry solution can vary from 0.5 to 13.5, generally being from 2to 5 in the acidic pH range and 8 to 11 in the basic pH range. The pH ofthe slurry can be adjusted by adding either inorganic acids or bases.Examples of strong inorganic acids include nitride acid, sulfuric acid,phosphoric acid, and hydrochloric acid. Examples of organic acidsinclude acetic acid, formic acid, and citric acid. Examples of inorganicbases include alkali (sodium, potassium, ammonium, lithium) basedhydroxides. Examples of organic bases include TMAH (Tetramethyl ammoniumhydroxide) and other hydroxides.

During polishing the hard material on the wafer surface is rubbed by apolymeric, metallic or a ceramic pad with the hard material having arelative velocity with respect to the polishing pad. This relativevelocity can vary from 0.01 m/sec to 100 m/sec with a typical range of0.2 m/s to 4 m/s. The pressure during CMP can vary from 0.1 psi to 100psi, with a typical range from 1 psi to 10 psi. Examples of polymericpads include polyurethane-based pads, and other polymeric materials thatgenerally all have a Shore D hardness of less than 100. The porosity ofthe pads can vary from 0.01% to 99% with a typical range from 10% to50%. The density of the pads can vary from 0.4 gm/cm³ to 1.0 gm/cm³.Examples of metal pads include cast-iron, copper, tin, and acopper-polymer composite. Examples of ceramic pads include silica,glass, alumina, sapphire pads and other ceramic materials with a Vickershardness exceeding 500 Kg/mm².

The buffering agent can provide several beneficial functions during thepolishing process. The buffering agent keeps the pH of the slurrysolution stable during the polishing process and also helps to increasethe polishing rate as compared to un-buffered oxidizers especially inthe pH range of 3 to 10. This is an unexpected result as conventionallythe addition of such additives reduces the polishing rate. A bufferingagent is defined herein as a material which increases the amount ofstrong acid such as nitric acid, sulfuric acid, or hydrochloric acidneeded to change the pH of the slurry solution from 9.0 to 3.0. Thebuffering ratio (referred to herein as the BR ratio) refers to amount ofstrong acid required when changing the pH of the slurry solutioncontaining the buffering agent as compared to the amount of strong acidrequired to change the pH from 9 to 3.0 of the slurry solutioncontaining no buffering agent. By adding a buffering agent to the slurrysolution the BR value can be between 1.1 and 200, with a typical BRvalue range from 2 to 20, such as from 2 to 10.

Examples of buffering agents include colloidal particles of softabrasives where soft abrasives are defined herein as an abrasive with aMohs hardness less than 4 or a Vickers hardness less than 300 Kg/mm²,organic compounds including polymers containing at least one hydroxyl(OH) group with a concentration ranging from 0.001 g/liter of persolution to 100 g/liter having a typical range from 0.05 gm/liter to 9gm/liter, and surfactants or surface active polymers having a molecularweight exceeding 100 Daltons with a concentration from 0.0001 g/liter ofper-compound to 100 gm/liter with typical concentration range from 0.1gm/liter to 5 gm/liter. Other buffering agents include mixtures ofstrong acids (examples nitric acid, hydrochloric acid, and sulfuricacid) or strong bases (sodium hydroxide, or potassium hydroxide) mixedwith weakly disassociating compounds which have pKa (the aciddissociation constant at logarithmic scale) ranging from 2.0 to 10.0such as citric acid, acetic acid, oxalic acid, phosphates, and borateswith the concentration of the weak disassociating compounds in theslurry solution varying from 0.1 gram/liter to 100 gram/liter such as0.5 gram/liter to 10 grams/liter.

The slurry solution can also include transition metal ions besides theper-based oxidizer such as manganese, copper, titanium, transition mixedmetal ions of manganese with valence varying from +2 to +7 withconcentration varying from 0.001 M to 10 M with typical range of 0.02Mto 0.3 M. The transition metal ions in the slurry solution together withtransition metal ions that may be in the per-compound oxidizer cantogether function as a buffering agent when including at least 2valences of manganese selected from 2+, 3+, 4+, and 7+ with totalmanganese ion concentration typically not exceeding 2.0 moles/liter.

A variety of surfactants can also be added to disclosed slurries as abuffering agent selected from cationic, anionic, zwitterionic ornon-ionic ones. The surfactants can be used individually or in a mixedstate. A list of surfactants that can be used with the invention areprovided in a book by M. J. Rosen, Surfactants and InterfacialPhenomena, John Wiley & Sons, 1989, hereinafter Rosen, on pages 3-32,52-54, 70-80, 122-132, and 398-401. The concentration of surfactants canvary from 0.0001 g/liter to 100 gm/liter with typical concentrationrange from 0.1 gm/liter to 5 gm/liter. The BR was found to change from1.0 to greater than 2.5 with the addition of a non-ionic surfactant.Furthermore the increase in the BR ratio also leads to an increase inthe removal rate during the polishing of hard metals and non-metals.

The non-ionic surfactants can comprise polyethylene glycol ethers,polypropylene glycol alkyl ethers, glucoside alkyl ethers, polyethyleneglycol octylphenyl ethers, polyethylene glycol alkylphenyl ethers,glycerol alkylesters, polyoxyehylene glycol sorbitan alkyl esters,sorbitan alkyl esters, cocamide, dodeceyldimethylamine oxide, blockcopolymers of polyethylene glycol and poly propylene glycol,polyethoxylated tallow amine. Examples of specific non-ionic surfactantsinclude TX-100 or BRIJ 35 (a polyethylene glycol dodecyl ether,Polyoxyethylene (23) lauryl ether). The concentration of the non-ionicsurfactants should generally be at least 0.001 mg/liter to a maximumvalue of 50 g/liter in solution, such as in a range from 0.03 gm/literto 5 gm/liter.

As describe above, the slurry solution can also include organiccompounds having OH groups compounds as the buffering agent. Examplesinclude organic acids, alcohols, amines (e.g., bicine, TEA) or compoundshaving a chemical formula represented by RCH₂OH where R, represent acarbon containing group such as H₂₁C₁₀—CH₂—CH₂—[C₂H₄]₂₂—O—CH₂— in thecase of a non-ionic surfactant (e.g. BRIJ-35) or containing from 3 to upto 70 carbon atoms. The organic compound concentration should generallybe at least 0.001 mg/liter up to a maximum value of 50 g/liter, such asa range of 0.1 gm/liter to 5 gm/liter.

As noted above the slurry solution can also include colloidal particlesof soft abrasives as the buffering agent. Examples include polymericparticles, zinc oxide, and manganese dioxide with particle size rangefrom 10 nm to 100 microns with typical range from 100 nm to 10 microns.The concentration of colloidal particles of soft abrasives shouldgenerally be at least 0.1 g/liter to a maximum value of 300 gm/liter ofslurry solution, with typical range from 0.1 gm/liter to 50 gm/liter.The soft slurry particles can have a negative zeta potential at a pH of6.0 or below.

The addition of buffering organic compounds such as having the formulaRCH₂OH together with per-compound oxidizers can lead to formation ofintermediate compounds in the slurry due to in-situ chemical reactions.An example of an in-situ chemical reaction is given below where RCH₂OHfunctions as a buffering agent:RCH₂OH+2MnO₄ ⁻+2H⁺→RCHO+2MnO₂+2H₂O+O₂where R is a carbon containing organic group. Due to this chemicalreaction the RCHO group may also act as buffering agent. Thus thepolishing slurry may contain an RCHO group in addition to a RCH₂OH groupthat may be formed in-situ. The RCHO groups formed in-situ in the slurrycan vary in concentration from 0.01 gm/liter to 100 gm/liter, such as0.1 gm/liter to 10 gm/liter, and can be formed at basic pH from 8 to 11or acidic pH from 2 to 5. The manganese oxide formed can be in form of aprecipitate, or can coat soft abrasive particle surfaces.

The slurry solution can optionally also include at least one alkalimetal ion (e.g., Li+, K+, and Na+) besides the per-based oxidizer. Thealkali metal ion in the slurry solution is generally in a concentrationfrom 0.01M to 10 M, with a typical range from 0.1 M to 0.5 M, orphosphate, acetate, sulfur or chlorine containing ions ranging inconcentration from 0.001 M to 10 M with typical concentrations rangingfrom 0.01 M to 0.5 M.

The polishing rate of a wide variety of different hard materials can beappreciable using disclosed embodiments despite excluding conventionallyneeded hard abrasive particles. It is recognized to be important toincrease the kinetics of the slurry's reaction with the hard surface toachieve high polishing rates. By adding at least one buffering agent thehard material polishing rate can be significantly enhanced. This is anunexpected result because buffering agents are known to tend to reducethe polishing rate, but disclosed slurries with such additives have beenfound to increase the hard material removal rate. It appears that thepresence of buffering agent(s) in the slurry solution tends to catalyzethe oxidation reaction of the hard layer being polished thereby leadingto significantly increased polishing rates.

The polishing process can take place using a CMP apparatus when thewafer surface is rubbed with a slurry by a polymeric pad or a metalplate or ceramic plate. The flow rate of the slurry can vary from 1ml/min to 10 liter/min with typical flow from 10 ml/min to 2,000 ml/min.The polishing pressure can vary from 0.1 psi to 20 psi with typicalrange from 1 psi to 10 psi. The linear velocity can vary from 0.01 m/secto 100 m/sec with typical range from 0.4 m/sec to 5 m/sec. Thetemperature of the slurry can vary from 5° C. to 80° C., with a typicalrange from 20° C. to 50° C.

FIG. 1 is a flow chart that shows steps for an example method 100 ofabrasive-free polishing of hard materials, according to an exampleembodiment. Step 101 comprises providing a slurry solution including ≥1per-compound oxidizer in a concentration between 0.01 M and 2 M with apH level from 2 to 5 or 8 to 11, and ≥1 buffering agent. The bufferingagent provides a buffering ratio of at least 1.5 which compares anamount of a strong acid needed to reduce the pH from 9.0 to 3.0 comparedto an amount of strong acid to change the pH from 9.0 to 3.0 without thebuffering agent. The slurry solution is exclusive any hard slurryparticles or has only soft slurry particles that have throughout aVickers hardness <300 Kg/mm² or Mohs Hardness <4. Step 102 comprisesdispensing the slurry solution on a hard surface having a Vickershardness >1,000 kg/mm². Step 103 comprises pressing with a polishing padwith the slurry solution on the hard surface in between while rotatingthe polishing pad relative to the hard surface.

EXAMPLES

Disclosed embodiments are further illustrated by the following specificExamples, which should not be construed as limiting the scope or contentof this Disclosure in any way.

Example 1

Experiments were performed using a CETR polisher from Bruker Corporationwith 9-inch platen, using a rotation of the pad at 100 rotations perminute (RPM) and the sample at 60 rotations per minute (RPM) by pressingagainst each other with a pressure of 6.3 psi. A soft polyurethane pad(Cabot D100) with a Shore D hardness of less than 100 was used for thepolishing process. A slurry with KMnO₄ as the per-compound oxidizer in aconcentration of 0.30 moles/liter dissolved in water with a pH 1 to 13was dispensed using peristaltic pump on the polishing pad. The slurrysolution was dispensed at 30 to 40 ml/minute.

The removal rates of silicon carbide and gallium nitride, and diamondlayers at different pH and concentration of the KMnO₄ solution are shownin the table provided in FIG. 2. This table clearly shows the highremoval rate of various carbides and nitrides by the permanganateslurry. An interesting feature of this polishing process is that removalrate goes up as the pH is increased in the acidic range. A significantresult for this polishing process is the result of a surface roughness 1to 2 Å (rms) with no subsurface damage was achieved. Furthermore, thishard abrasive free composition can contain small particles of MnO₂formed due to autocatalysis. The Mohs hardness for MnO₂ is ≤3. Thus twostates of manganese ions (+7, +4) and dissolved manganese (Mn²⁺) may allbe present in the slurry solution. No hard abrasive particles were addedto the slurry which means that no abrasives with Mohs number >3.0 orVickers hardness >300 kg/mm² were present. It is interesting to notethat high removal rates are obtained for the hard materials polished(Vickers hardness of GaN 1500 Kg/mm², SiC approximately 3,000 Kg/mm²,and diamond approximately 10,000 Kg/mm²) despite lacking conventionallyrequired hard abrasives in slurry. The removal action is based upon aformation of a modified (oxidized layer) on the surface of the hardmaterial layer which is removed by the rubbing of the pad. Applicantshave seen removal or polishing of carbide/nitride surfaces when usingpolymeric pads with Shore Hardness D varying from 5 to 100. The removalrate was found to be linearly dependent on pressure up to 10 psi andrevolution rate (10 to 300 rpm).

Example 2: Effect of Pad Pressure

Experiments were performed using a Buehler polisher with 12-inch platen,rotating the pad at 90 RPM and sample with 60 RPM and by pressingagainst each other with pressure varying pressure. A slurry with KMnO₄as the per-compound oxidizer in a concentration of 0.1 mole/literdissolved in water at a pH of 2 was dispensed during the polishingexperiment using peristaltic pump. A polyurethane pad (Cabot D100) wasused for this polishing process with Shore D hardness of approximately40 The removal rates of c-face SiC wafers with different pad pressuresusing the above-mentioned process is shown in the table below. It shouldbe noted that polymeric pads with a Shore D hardness ranging from 10 to100 or Shore A hardness ranging from 5 to 100 are expected to give riseto high polishing rates for such slurries.

SiC—C— face@ pH~2 Pressure psi Removal rate (nm/h) 0.5 350 2.0 2,4006.36 5,450 9.55 7,880 12.73 9,880 15.92 10,730

The SiC removal rate was found to be approximately linear with pressure.The removal rate of c-face SiC was found to be much higher than Si-face.It is believed that these good results are because of strong interactionof the permanganate ions with carbon and nitrogen based bonds, with suchbonds being susceptible to oxidation of the surface.

Example 3: Effect of Temperature

Experiments were performed using a Buehler polisher with 12-inch platen,rotating the pad at 90 RPM and the sample at 60 RPM by pressing againsteach other with a pressure of 6.3 psi. A slurry with KMnO₄ as theper-compound oxidizer in a concentration of 0.4 mole/liter dissolved inwater at a pH of 2. The solution was heated to different temperaturesusing a hot plate. The heated solution at different temperatures wasdispensed during the polishing experiment using a peristaltic pump. Apolyurethane pad (Cabot D100) was used for this polishing process. Theremoval rates c-face SiC wafers with different temperatures using theabove-mentioned process is shown in the table below. It should be notedthat polymeric pads with Shore D hardness ranging from 10 to 100 orShore A hardness ranging from 5 to 100 are expected to give rise topolishing rates for such slurries.

SiC—C— face@ pH~2 Temperature Removal rate (μm/h) 25° C. 7.3 50° C. 7.7

The polishing temperature can vary from 10° C. to 50° C., however nostrong temperature dependence was observed.

Example 4: Effect of Salt Addition

Experiments were performed using a Buehler polisher with 12-inch platen,rotating the pad at 90 RPM and the sample at 60 RPM and by pressingagainst each other with a pressure of 6.3 psi. A slurry with KMnO₄ asthe per-compound oxidizer in a concentration of 0.05 mole/liter anddifferent (organic and inorganic) salts with different concentrationswas dissolved in water at a pH of 1.6. This mixed slurry solution wasdispensed on the pad during the polishing experiment using a peristalticpump. A polyurethane pad (Cabot D100) was used for this polishingprocess. The removal rates C-face SiC wafers with different saltaddition, using the above-mentioned process is shown in the table below.

SiC—c— face@ pH 1.6 Salt Salt Concentration Removal rate (nm/h) No salt0 4,506 NaCl  0.2 mol 3,618 KCl  0.2 mol 3,656 K₂SO₄  0.1 mol 3,900 KNO30.15 mol 4,205 Na₂HPO₄ 0.01 mol 3,560 CH₃COONa 0.01 mol 3,500

The above salts were found to decrease the removal rates, however abetter uniformity in polishing was observed.

Example 5: Effect of Pads

Experiments were performed using a Buehler polisher with 12-inch platen,rotating the pad at 90 RPM and the sample at 60 RPM by pressing againsteach other with a pressure of 6.3 psi. A slurry with KMnO₄ as theper-compound oxidizer in a concentration of 0.1 mole/liter dissolved inwater at pH of 2 was dispensed on the pad during the polishingexperiment using a peristaltic pump. The experiments were performedusing polishing pads made of polyurethane, poromeric, copper composite(copper and epoxy) and metals places copper and cast iron. The poromericpad is composed of a polymer with Shore hardness A of less than 20. Theremoval rates C-face SiC and Ga-face GaN wafers with use of differentpad type using the above-mentioned process is shown in the table below.

SiC—C— face@ pH~3, GaN Ga-face @ pH~1.5, Pad Removal rate (μm/h) Removalrate (μm/h) Polyurethane 5.38 0.3 Poromeric pads 7.7 0.25 CopperComposite 0.66 0.15 Copper plate 0.55 0.1 Cast Iron 0.66 0.1

The hardness of the polymeric pads can be varied from Asker C-20 toAsker −100 and shore D hardness from 10 to 100. Substantial removal ofmaterial was observed. Cast iron and copper plates were found to givelower removal rates compared to pads. Corrosion inhibitors such asamines and azoles (benzo triazole (BTA) were added to the slurry toreduce its corrosive properties but it did not significantly decreasethe polishing rates.

Example 6: Polishing of Different Materials

Experiments were performed using a CETR-CP-4 polisher with 9-inchplaten, rotating the pad at 100 RPM and the sample at 60 RPM by pressingagainst each other with a pressure of 6.3 psi. A slurry with KMnO₄ asthe per-compound oxidizer in a concentration of 0.03 to 1.5 mole/literdissolved in water at a pH of 2 was dispensed on the pad during thepolishing experiment using a peristaltic pump. An IC 1000 polishing padfrom DOW Electronic materials comprising a micro-porous polyurethanematerial was used. The experiments were performed using different hardsubstrates and the removal rates achieved on each substrate is shown inthe table below.

Oxidizer Concentration Removal rate Material (mole/liter) pH (μm/h)AlGaN 0.05 2-3 0.35 Al-85%, Ga-15% 0.50 2-3 1.7 AlN 1.50 2-3 0.95Ruthenium (Ru) 1.5 2 0.1 Ruthenium (Ru) 0.5 3 0.12 Tungsten (W) 0.1 20.06 Iridium (Ir) 0.1 2 0.05 Tantalum (Ta) 0.1 4 0.02

This data shows that a KMnO₄ slurry without hard abrasive particlesprovides significant removal of material from both AlGaN and metalsurfaces.

Example 7 Effect of Additives

Experiments were performed using a Buehler polisher with 12 inch platen,rotating speed of the pad at 90 RPM and the sample of on axispoly-crystalline silicon carbide at 60 RPM by pressing against eachother with a pressure of 6.3 psi. A slurry with KMnO₄ as theper-compound oxidizer in a concentration of 0.3 mole/liter was dissolvedin water at a pH of 3 was mixed with different surfactants,concentration of organic compounds with OH groups and Mn²⁺ ions wasdispensed on the pad during the polishing experiment using a peristalticpump. The experiments were performed using different substrates and theremoval rates with each additive normalized to a 0.25 mole/liter KMnO₄solution is shown in the table below.

Removal rate 0.25 mol/liter KMnO₄ Additive/Concentration pH solution0.25 mole/liter KMnO4 1.5 0.66 0.25 mole/liter KMnO4 3 0.5 0.25mole/liter KMnO4 5 0.45 Cationic Surfactant: Cetrimonium 3 0.7 bromide(C12TAB) (0.01 g/L) Cationic Surfactant: Cetyl ammonium 3 0.58 bromide(C12TAB) (5 g/L) Non-ionic surfactant: Pentaethylene 9 0.75 glycolmonododecyl ether (0.2 g/L) (Brij-35) Non-ionic surfactant Pentaethylene3 0.87 glycol monododecyl ether (1 g/L) (Brij-35) Non-ionic surfactant:Octaethylene 3 0.69 glycol monododecyl ether (0.3 g/L) (Brij) Non-ionicsurfactant: Octaethylene 3 0.73 glycol monododecyl ether (5 g/L) KMnO4(Brij-35) Anionic Surfactant: Sodium dodecyl + 3 0.71 0.12 g/L) AnionicSurfactant: Sodium dodecyl + 3 0.61 4 g/L) Non-Ionic surfactantPolyethylene 10 0.33 glycol octylphenyl ethers. TX100 + 4 gm/L Non-Ionicsurfactant Polyethylene 3 0.77 glycol octylphenyl ethers. TX100 + 0.1gm/L Non-Ionic surfactant Polyethylene 5 0.93 glycol octylphenyl ethers.TX100 + 0.1 gm/L Bicine N,N-bis (2-hydroxyehyl) 7.5 0.57 1 glycine 1g/L) Bicine N,N-bis (2-hydroxyehyl) 3 0.85 glycine 0.5 g/L) BicineN,N-bis (2-hydroxyehyl) 3 0.69 glycine 1 g/L) 0.03 mole ManganeseChloride 3 0.62 Hexahydrate (MnCl2•6H20)

Various additives with different concentration were added for 0.25 moleKMnO₄ disclosed slurries. The removal rates were found to decrease withadditives at lower pH but appear to be the same or higher at higher pH(e.g., pH ˜5). As disclosed above the BR is defined as the strong acidrequired to reduce pH from 9 to 5 when compared to the same slurry withno buffer agent added. For non-ionic surfactants, the BR was between 1.0and 10.0 depending on the concentration of the non-ionic surfactant. Theability to have a high BR is advantageous as it leads to constant pHalong the polishing process. The addition of organic additives such asbicine as a buffering agent with surfactants seems to increase thebuffering ratio. With bicine a BR of 1.2 was observed (0.05 gm/liter)while a BR of 5.6 was obtained with TX-100 (a non-ionic surfactant thathas a hydrophilic polyethylene oxide chain and an aromatic hydrocarbonlipophilic or hydrophobic group). In this case a higher removal rate wasobtained.

Example 8 Effect of Multiple Manganese Ion Valence States

Experiments were performed using a Buehler polisher with 12 inch platen,rotating speed of the pad at 90 RPM and the sample being on axis 4Hsilicon carbide Si-face wafer at 60 RPM by pressing against each otherwith a pressure of 6.3 psi. A slurry with KMnO₄ as the per-compoundoxidizer in a concentration of 0.3 M was dissolved in water at a pH of 3mixed with soft oxides of manganese oxide (such as MnO₂) with a Vickershardness less than 200 Kg/mm² or Mohs hardness less than 3 with chargesstates Mn²⁺, Mn³⁺, Mn⁴⁺ and Mn⁷⁺ that was dispensed on the pad duringthe polishing experiment using a peristaltic pump. A polyurethane pad(Cabot D100) was used for this polishing process. The experiments wereperformed using different substrates and the removal rates with eachadditive normalized to 0.3 mole/liter of KMnO₄ solution is shown in thetable below.

Removal rate 0.3 mol/ 0.1 mole/liter KMnO₄ pH liter KMnO₄ solution 0.1mole/litter KMnO₄ 3 0.2 1 gm/liter of Manganese(II) oxide, 3 0.2 MnO 1gm/liter of Manganese(III) oxide 3 0.201 1 gm/liter of Manganese(IV)oxide 3 0.23 1 gm/liter of Manganese(VII) oxide, 3 0.23 Mn₂O₇

These results show that addition of Mn⁺⁴ increases the rate of polishingprocess. The presence of multivalent manganese ions also can be seen tohelp the polishing process. It should be noted that polymeric pads withShore D hardness ranging from 10 to 100 or a Shore A hardness rangingfrom 5 to 100 are expected to give rise to polishing rates for suchslurries.

While various embodiments of the invention have been described above, itshould be understood that they have been presented by way of exampleonly, and not limitation. Numerous changes to the disclosed embodimentscan be made in accordance with the disclosure herein without departingfrom the spirit or scope of the disclosed embodiments. Thus, the breadthand scope of embodiments of the invention should not be limited by anyof the above explicitly described embodiments. Rather, the scope of theinvention should be defined in accordance with the following claims andtheir equivalents.

The invention claimed is:
 1. A slurry for chemical-mechanical polishing(CMP), comprising: an aqueous medium; at least one pre-compoundpermanganate oxidizer that includes an element in its highest oxidationstate with a concentration between 0.01 M and 2.0 M; a pH level from 8to 11; at least one buffering agent different from said per-compoundoxidizer, the buffering agent comprising at least one of a surfactantand an alkali metal ion; and particles of MnO₂ having a Mohs hardness of≤3, wherein said slurry is exclusive of any particles that have a Mohshardness >3.
 2. The slurry of claim 1, wherein said slurry furthercomprises transition metal ions in a concentration from 0.03 M to 1 M inaddition to any transition metal ions that may be in said per-compoundpermanganate oxidizer.
 3. The slurry of claim 1, wherein said particlesof MnO₂ having a Mohs hardness ≤3 are particles formed in-situ byautocatalysis.
 4. The slurry of claim 1, wherein said buffering agentcomprises said surfactant and at least 2 different valence states of Mnions including said per-compound permanganate oxidizer chosen from +7,+4, +2, and +3.
 5. The slurry of claim 1, wherein said per-compoundpermanganate oxidizer comprises potassium permanganate or sodiumpermanganate.
 6. A slurry for chemical-mechanical polishing (CMP),comprising: an aqueous medium; at least one pre-compound permanganateoxidizer that includes an element in its highest oxidation state with aconcentration between 0.01 M and 2.0 M; a pH level from 2 to 5 or from 8to 11; at least one buffering agent different from said per-compoundoxidizer, the buffering agent comprising at least one of a surfactantand an alkali metal ion; and particles of MnO₂ having a Mohs hardness of≤3, wherein said slurry is exclusive of any particles that have a Mohshardness >3, and wherein the buffering agent comprises a species havingthe formula RCHO, wherein R is a carbon containing group.